Method of measuring pressure with a magnetron



Oct. 12, 1954 l. R. SENITZKY METHOD OF MEASURING PRESSURE WITH A MAGNETRON 2 Sheets-Sheet 2 Filed Jan. 26, 1951 Fig.5.

\wwtmqtw SEER kEMkwSU MQQEY /0' (MM 0/: MERCURY) PRESSURE UNCOATED 047M905 ANODE VOL TAGE =50 d J m m m \wtvsq w g fiQtmwmrk 10* 10' (MM 0F MERCURY) PRESSURE wwmmw \wwkmqvxw I Shit kRwtkSb ESE R. F. POWER INVENTOR. mun R. szsw/rz/rr ATTORNEY Patented Oct. 12, 1954 UNITED" OFFICEv METHOD OF MEASURING PRESSURE WITH A. MAGNETRON (Granted under Title sec. 1.

The invention described herein may be manuzfactured and used by or: tor the: Government for governmental purposes, without the payment to me of any royalty thereon.

My invention: relates; to a novel method of measuring low gas; pressures; particularly pressures as low as millimeter of mercury.

It has been found that under certain conditions, to be more fully described, an anode?- cathode current can be caused to flow in a cold cathode resonant ca-vity magnetron. The magnitude of this current depends upon numerous factors, including. thegas" pressure within the tube, the anode voltage,.and.the amount of radio frequency power introduced withinthe cavity.

Further, there is a minimum value.- of radio frequency power necessary to-maintaintheanode current. This minimum, or threshold value, variesv with the gas pressure within the tube.

The object of my invention is to utilize the above-described variation in current with pressure and radio frequency power. toprovide a method of. measuring low gas pressures by means of a cold cathode resonant cavity magnetron.

In accordance with apreferred embodiment of my invention, I employ a cold cathode resonant cavity microwave magnetron. A l'ow'positive potential, of the order of 50 or 100 volts, is applied to the anode with respect to the cathode. An axial magnetic field, whose magnitude. is close to the cyclotron value, as determined by the resonant frequency, is applied. Radio frequency power at the resonant frequency is supplied from an external source and introduced into the tube. As a result, a. steady anode-cathode current flows.

For a more complete understanding. of' my invention, reference maybe had to the accompany ing drawings, in which Fig. I is a cross sectional View of a magnetron suitable for use in the inventi'on; Fig. 2 is a schematic showing of the" entire apparatus; Figs. 3,.- 4 and 5 are. curves obtained as a result of'experi'ments, Fig. 3 showing the variation of? anode current with. gas pressure, Fig. 4 showing. the minimum R. F. power necessary to sustain the discharge atvarious pressures, and Fig. 5 showing the variation of anodecurrent with. applied R. F. power.

As shown in. Fig. 1, the tube, designated generally by the numeral I'D, is essentially a microwave magnetron. The anode block H has a number of identical slots [21 arranged symmetrically. The cathode 13' maybe coated or uncoatedl. A. conventional cathode heater (not shown-) is provided but: normally is not used. Radio frequency power is introduced. into: the

35.11. s. Gode (-1952), 266) cavity between the cathode l3 and the anode II by means of a section. of waveguide M which leads into the back of one of the slots [2. The transformer I5 and the window (not shown) are of cenventionai construction and may be, for example, of. the typeshown at page 497 of the book Microwave Magnetrons? by Collins, McGraW- Hill; 1948i.

It has been found that if a cold cathode magnetron. such as above described be subjected to certain critical conditions that a current will flow between the: anode and the cathode. These critical conditions are 1-.- R. F: powen. of. the order of microwatts, the frequency of which. is the resonant frequency of the tube; is incident on the tube.

2. An axiali magneticfield, the magnitude of which. is within a certain: critical rangein the neighborhood of. the cyclotronfield, is applied. (The cyclotron: field: is that value of field necessary' toproduce the welhknown cyclotronoscillations in the usual magnetron.)

3. A) positive voltage is applied totheanode with. respect to the cathode.

Upon application of. the critical-.-conditi'ons,. the

following effects may be: observed:

1.- After a period. or time which may vary from a fewseconds too; few minutes, anode current, at the order of microamperes, starts to flow. The amountv of time delay can be reduced by heating thecathod'e'. Once the anode current begins to flow; the heater current may be turned off without interrupting the anode current. The

amount of time delay depends on the previous history of the tube, being greater when the tube has been inactive for some time.-

2. The: anode current: is a function of the gas pressure within the tube,v as shown: in. Fig. 3.v

3."1 he anode current flowsonly if the magnetic' field. is: within a certairr criticala rangein the neighborhood or the cyclotron field.

4t There is a lowerlimit tothe anode voltage for which. anode current will flow.

Sr-Anodecurrentin'creases with an increase in Rl-Flpower. There'is' a critical or threshold, Value: of Ra F:v power, varying withipressure, below' which: the-anodecurrent wil-L not flow. Fig. 4' isa curve showing this threshold. power as a function or} pressure The: production ofi the anode current which flows uponv application ofthe critical conditions is calledthecyclotrom effect; The following explanation: is offered.

Consider an electron somewhere im the interaction space, subject to." the: critical: conditions.

It will absorb power from the R. F. field and accelerate. The trajectory is such that before the electron reaches an electrode, it traverses a very long path, which is comparable in length to the mean free path of an electron in a gas at the pressure which exists in the tube. If the electron has acquired energ-y equal to or greater than the ionization potential of the residual gas, there is a likelihood that it will ionize a molecule. After the ionization, there are two free electrons available to start the process over again, and so the ionization increases. The electrons drift slowly toward the anode. The positive ions absorb power from the D. C. field and reach the cathode from which they liberate electrons. Depending upon the type of cathode surface, there may also be, at room temperature, some thermionic emission. The electrons at the cathode undergo a process similar to that described above for the initial electrons and so a steady anode current is obtained.

Fig. 2 shows the arrangement of the apparatus. A. D. C. potential is applied between anode ii and cathode :3, the source being schematically shown as a battery 16 and a potentiometer ll. Anode current may be measured by a suitable instrument such as a microammeter 18. An axial magnetic field is provided by means of an electromagnet whose field coil 19 is supplied by a suitable adjustable source such as a battery 26 and a rheostat 21. R. F. power at the resonant frequency of the magnetron I is provided by a suitable source 22, such as a klystron oscillator. This R. F. power is fed to the resonant cavity of the magnetron iii. A calibrated R. F. attenuator 23 is inserted in the line between the source 22 and the magnetron W. A switch 24 is provided so that the attenuator may be selectively inserted into or removed from the circuit. A wave meter 25 may be employed to measure the frequency of the power from the source 22. A monitor 26 to measure the power is provided.

In order to measure low gas pressures, the space 27, whose pressure is to be measured, is connected through a shut-off valve 28 to the interior of the magnetron iii. The connection may be made in any suitable manner. One manner which has been found to be satisfactory is to run a tubulation through the pole piece of the electromagnet. If desired, a pump 29 may be connected to the space 21 to maintain the low pressure. For the particular tube for which the curve of Fig. 3 was drawn, the R. F. power is at 8900 megacycles. Switch 24 is opened and the R. F. attenuator 23 is set so as to apply 150 microwatts to the tube ill. The critical magnetic field is about 3100 gauss; Anode voltage, which is not critical, is set to 50 volts. Microammeter i8 is read, and the pressure determined from the curve of Fig. 3. If desired, meter 18 can be calibrated to read pressure directly.

The above method is limited to measuring comparatively high pressures. One reason for the limitation is the inability of ordinary instruments to measure the low currents involved. A second method utilizes the fact that there is a minimum value of applied R. F. power necessary to maintain the anode current. This minimum, or threshold value, varies with pressure, as shown in Fig. 4. The apparatus is set up as above, except that the R. F. power is decreased by means or the attenuator 23 until the anode current is interrupted. The pressure is determined from the curve of Fig. 4. At extremely low pressures interruption of the anode current cannot be observed with ordinary instruments. In order to note the minimum power necessary to sustain operation, use is made of the fact that once operation ceases, there is a time delay before it starts again. The R. F. power is, therefore, reduced in successive steps from some value at which anode current can be read conveniently, being returned to its initial value after each reduction. If anode current reappears immediately, it is an indication that the discharge has not been interrupted, while if it does not reappear immediately, the power has been dropped below the minimum necessary to sustain the discharge. The curve of Fig. 4 was plotted from experimental data. By means of the above technique and by extrapolation of the curve, pressures as low as 10- mm. of mercury have been determined.

It is sometimes desired to couple a source of R. F. power to a device whose R. F. impedance is adjustable. The apparatus used is that shown in Fig 2. The tubulation leading to space 26 is closed by any suitable means, such as the valve 28. If desired, a sealed off magnetron may be used. The anode voltage and magnetic field are applied as before. Switch 24 is closed to remove attenuator 23 from the circuit. The impedance seen by the source 22 may be varied by adjusting the anode voltage.

In order to detect small amounts of R. F. power, the apparatus is set up as explained above. Valve 28 is closed, as before, or a sealed off tube is used. Switch 24 is closed to remove attenuator 23 from the circuit. The presence of an anode current indicates that R. F. power at the resonant frequency is incident upon the tube. Fig. 5 shows the relationship between anode current and R. F.

power. Experimentally, an anode current has been sustained with an R. F. power as low as 10* watts.

It is to be understood that many variations in the structure and arrangement of parts may be made. For example, the particular anode structure shown in Fig. 1 is merely illustrative. Any other type of structure resonant at the same frequency may also be used. Similarly, the R. F. power may be introduced 'by any other suitable means, such as a coupling loop, instead of by means of the particular wave guide structure illustrated. The cathode may be coated or uncoated. At present, an uncoated cathode or seamless nickel tubing is preferred for measuring low pressure and for detecting small amounts oi microwave power, while an oxide coated cathode is preferred for providing a variable impedance. The electromagnet may be replaced by a permanent magnet providing the same field strength. Many other modifications within the scope of my invention will suggest themselves to one skilled in the art.

The term cold cathode as used herein means a cathode which is not heated directly from an external source, as distinguished from the usual thermionic cathode which is heated by an external source provided for that purpose.

I claim:

1. The method of measuring low gas pressures by means of a cold cathode resonant cavity magnetron comprising the steps of connecting the interior of the magnetron to the space whose pressure is to be measured; applying to the magnetron an axial magnetic field whose magnitude is substantially that of the cyclotron field; applying a positive potential, below the cut-off value, to the anode with respect to the cathode; introducing suflicient radio frequency power at the resonant frequency into the cavity to initiate a measurable anode current; reducing the amount of radio frequency power successively in increasingly large steps; returning the power level to its original value after each reduction until the anode current fails to reappear immediately; whereby the least amount of power, which allows the anode current to reappear immediately when the power level is restored, is a measure of the pressure within the space.

2. In a method of measuring low gaseous pressures by means of a cold cathode resonant cavity magnetron, the steps of connecting the space whose pressure is to be measured to the interior of said cold cathode resonant cavity magnetron, applying an axial magnetic field the magnitude of which is within a certain critical range in the neighborhood of the cyclotron field; applying a positive potential below the cut-on" value to the anode of the magnetron with respect to its cathode; introducing radio frequency power at the resonant frequency into the magnetron, whereby a steady anode-cathode current flows; and reducing the amount of radio frequency power until the minimum value of said radio frequency power necessary to maintain the anodecathode current at its threshold value is determined, the amount of power introduced at the instant the anode-cathode current is at said threshold value being a measure of the pressure within said space.

3. A method for measuring low gaseous pressures by means of an apparatus which comprises a cold cathode resonant cavity magnetron, said magnetron having an axial magnetic field incident thereon whose magnitude is substantially equal to the cyclotron field value, said magnetron also having a positive potential below the cutofi value applied to the anode, said method comprising the steps of connecting the space whose pressure is to be measured to the interior of said cold cathode resonant cavity magnetron, introducing sufiicient radio frequency power into the cavity of the magnetron to cause a steady anode current to flow, and reducing the amount of radio frequency power until the minimum value of said radio frequency power necessary to maintain the anode current at its threshold value is determined, the amount of power introduced at the instant'the anode current is at said threshold value being a measure of the pressure within said space.

4. A method for measuring low gaseous pressure by means of a resonant cavity cold cathode magnetron operating with an anode voltage below cut-off and a magnetic field substantially equal to the cyclotron field value, the interior of the magnetron communicating with the space whose pressure is to be measured, comprising the steps of applying sufficient radio frequency power, at the resonant frequency, to the magnetron to cause a measurable steady anode current to flow; reducing the amount of radio frequency power and immediately returning the power to its original value; successively repeating the foregoing step with increasing reductions in the amount of power until the anode current fails to reappear immediately, whereby the least amount of power which sustains the anode current is a measure of the pressure within the space.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,081,429 Gaede May 25, 1937 2,197,079 Penning Apr. 16, 1940 2,423,716 McArthur July 8, 1947 2,448,527 Hansell Sept. 7, 1948 2,513,933 Gurewitsch July 4, 1950 2,562,738 Ramo July 31, 1951 OTHER REFERENCES FM and Television, Fisk et 211., April 1947, pp. 41 and 42, Scientific Lib. 

